1. Field of the Invention
The present invention relates to an exposing method which is used to form various devices used in a semiconductor chip, a displaying device, a detecting device, an imaging device, an electron-emitting device, micromechanics, a photonic crystal, and the like. In particular, the present invention relates to an exposing method which is suitably used as an exposing method of forming a periodic two-dimensional dot pattern to an object to be processed in a photolithography process.
2. Description of the Related Art
A projection exposing apparatus which transfers a pattern by projecting a mask (or reticle) pattern to a wafer via a projection optical system with use of photolithography technique has been conventionally used. Here, a resolution (i.e., a line width of mask) R of the projection exposing apparatus is given by the following Rayleigh expression with use of a wavelength λ (m) of a light source and a numerical aperture NA of the projection optical system.R=k1×λ/NA  (1)
On the other hand, a range of focus capable of maintaining certain image formation performance is called a depth of focus, and the depth of focus DOF is given by the following expression.DOF=k2×λ/NA2  (2)
Here, if the depth of focus DOF becomes small, it becomes difficult to perform focusing, and it is thus required to improve flatness (evenness) of a substrate and focusing accuracy. For this reason, it is basically desirable to enlarge the depth of focus DOF.
In recent years, since it has been requested to make a device minute, a demand for stably resolving a micropattern by the projection exposing apparatus has increased so much more. Here, to achieve stable high-resolution, it is necessary to select optimum exposing conditions (a kind of mask, an illumination condition, and the like) according to a kind of pattern. Further, since it is necessary to enlarge the depth of focus in order to stabilize the image formation performance, the illumination condition affects the depth of focus.
For example, Japanese Patent Application Laid-Open No. 2007-109969 (called a patent document 1 hereinafter) discloses an exposing condition which is optimized in regard to dots or holes having two-dimensional periodicity such as contact holes of a semiconductor device, Spindt-type field emission devices, micromechanics, photonic crystals, and the like. More specifically, the patent document 1 discloses an exposing method which can achieve both resolution and a depth of focus by applying hexa-pole illumination to a mask on which apertures are zigzaggedly arranged.
In the exposing method which obtains a two-dimensional periodic pattern on an object to be exposed, it is generally difficult to obtain resolution as compared with an exposing method which obtains a one-dimensional periodic pattern, because of the following reasons. That is, since it is necessary to capture diffracted light of at least three or more light fluxes on a pupil in order to obtain the two-dimensional periodic pattern, an interval between the respective light fluxes is narrower than that in case of the one-dimensional periodic pattern. In addition, since the number of the light fluxes increases, it is difficult to obtain a contrast on an image surface.